DESCRIPTION: Type 1 diabetes (T1D) is an autoimmune disease caused by immune destruction of insulin-producing pancreatic cells. T1D affects 3 million children and adults in the US with healthcare costs exceeding $14.9 billion. Allogeneic islet transplantation has proven effective in improving metabolic control/quality of life and in preventing severe hypoglycemia in patients with T1D. However, broad clinical application of allogeneic islets is limited by i) immune rejection despite the chronic use of immunosuppression and its sequelae, ii) transplantation into the liver, which compromises immediate post-graft islet engraftment as well as long-term survival, and iii) insufficient human islet supply from cadaveric pancreata. Thus, there is a significant need for innovative strategies that overcome these limitations of islet transplantation for a broader clinical practice. This R21 application is formulated to overcome graft rejection without chronic use of immunosuppression and establish the small bowel mesentery as an extrahepatic site for islet transplantation, thereby overcoming two critical barriers of islet transplantation. This will be achieved by engineering dual-action immunomodulatory and vasculogenic hydrogels for controlled delivery of a novel form of PD-L1 and VEGF for islet transplantation in the small bowel mesentery. PD-L1 (programmed death-ligand 1) is an important immunoregulatory molecule involved in tolerance to self-antigens and has extensively been exploited by chronic infections and cancer for immune evasion. PD-L1 interaction with PD-1 receptor on various adaptive and innate immune effector cells results in inhibition of effector function and/or physical elimination of these cells. Importantly, PD-1/PD-L1 interaction is involved in the generation of tolerogenic DCs and induced CD4+CD25+FoxP3+ Treg cells that are critical to peripheral tolerance to self-antigens and acquired tolerance to foreign antigens in various experimental settings. VEGF has recently been shown to confer a stable tolerogenic phenotype to DCs by inducing the expression of indoleamine 2,3-dioxygenase (IDO). IDO is an important immunoregulatory enzyme and signaling molecule that is involved in fetomaternal tolerance and generation of Treg cells. Our central hypothesis is that PD-L1 and VEGF delivered by biomaterials will work in synergy to induce localized tolerance and promote islet vascularization for long-term allogeneic islet survival and function without continuous use of immunosuppression. The overall objective will be accomplished by testing our central hypothesis through two Aims: 1) Engineer PD-L1/VEGF- presenting hydrogels to generate Treg cells in vitro and facilitate allogeneic islet engraftment, vascularization, and survival in short-erm in vivo studies; and 2) Test the efficacy of the PD-L1/VEGF hydrogels to support long-term engraftment, vascularization, tolerance induction, and treatment of diabetes. Studies will be conducted to elucidate the mechanistic basis of graft failure and/or long-term survival with particular focus on innate and adaptive effector immune and regulatory (tolerogenic dendritic cells and Treg cells) responses.